A semiconductor light-emitting device (LED) is one type of optoelectronic device that utilizes a contact for a group III nitride semiconductor heterostructure. Light extraction from an LED is limited due to total internal refraction of the light within the material of a light generating structure, active region or light-emitting layer used with the group III nitride semiconductor heterostructure. Typical refractive indices for group III nitride semiconductor devices are in the range of 2.2 to 2.7. The amount of extraction depends heavily on the macroscopic geometry of the LED and the three-dimensional emission profile of light generated within the light generating structure. Before it can escape, most of the light generated within the device is attenuated by the absorbance in the surrounding materials, e.g. epitaxial layers, confining regions, substrate, die attach materials, and electrical contacts.
Typical devices generate photons at the p-n junction that are emitted into a wide range of directions (nearly isotropic emission). As a result, a large percentage of emitted light rays are incident at a device/ambient interface at angles greater than a critical angle for exiting the semiconductor heterostructure. These rays are internally reflected and are susceptible to absorption within the device. For a typical AlGaN-based semiconductor heterostructure of an LED, only a small fraction of photons are incident on the top surface within the critical angle for transmission into epoxy. The remaining light undergoes at least one internal reflection before escaping the chip.
The internally-reflected light in AlGaN LEDs is also susceptible to absorption by a p-layer contact. For ultraviolet LEDs, a metal contacting a p-type semiconductor contact is typically poorly reflective. In some instances, aluminum has been used with a p-type semiconductor contact to improve reflectivity, however, aluminum does not form a good Ohmic contact to a p-type metal.